The present invention relates to the field of metrology, and, more particularly, to scanning probe microscopy.
Scanning probe microscopes (SPM) utilize a very fine probe having a tip which is maintained either just touching or very close to the surface of a sample. The probe is mounted to a scan head and the sample is mounted to a table, with the scan head and table being arranged for three-dimensional relative movement therebetween. To obtain an accurate profile of a feature on the nano-scale, the sample surface cannot be altered by the measurement and the probe tip or stylus shape must be known and cannot change during the measurement.
Stylus NanoProfilometry (SNP) technology addresses identified shortcomings of cantilever-based atomic force microscopy (AFM) instruments, namely the constantly changing geometrical relationship between the tip and the sample, tip wear, accretion of contamination, and constantly varying tip-to-sample geometry. An example of a stylus nanoprofilometer is the S/II offered by Surface/Interface, Inc. of Sunnyvale, California. SNP achieves control by using characterized tip shapes with on-board tip characterizers, controlling the force of the sample tip interaction and the orientation of the tip with respect to the sample plane, and interacting with the surface only once for each data point obtained (pixel-only sampling). SNP uses computer algorithms to calculate the sample profile from the scan output by accounting for the contribution of the tip shape to the raw data.
As it is implemented in a metrology tool, SNP combines traditional scanning probe microscopy and stylus profilometry with automatic tip characterization and new force-controlled and angle-controlled sensing technologies (i.e., controlled-force contact and angle-controlled contact), adding new capabilities to submicron and deep submicron critical dimension (CD) metrology for semiconductor wafers and masks. This nondestructive technique obtains and displays profiles of lines, trenches, and other features and quantitatively characterizes critical sidewall parameters such as slope and profile shape at top and bottom corners without cross-sectioning.
SNP profiles can be obtained at multiple locations along a line or in a trench, adding statistics to CD measurements and information on line-edge roughness. SNP can also be a calibration partner for CD scanning electron microscopes (CD-SEMs), replacing the physical cross-sectioning procedure required to check CD-SEM measurements. Compared to SEM imaging of a line or trench""s physical cross-section, SNP turnaround time is short since measurements can be done in a wafer fabrication plant instead of an off-line analytical lab. SNP technology allows for multiple, sequential scans to produce data used to build detailed images along a feature or line. An additional benefit of SNP is that multiple profiles can be taken along a line or trench to see nanoscale detail that could have significant process ramifications.
Because the tip geometry needs to be subtracted from measurement data to determine the true profile of the measured structure, all aspects of the tip-to-sample interaction must be fully characterized and controlled. Any changes to tip geometry from wear dramatically affects the final results. Damage or accretion of contamination on the scale of a few nanometers will significantly degrade the final profile obtained. It is also critical to have accurate measurement and control of the force between the tip and the measured structure, even when the force is attractive. In contrast to a cantilever AFM, in which the probe tip continuously scans and touches (or nearly touches) the measurement point, SNP uses a step-and-repeat measurement sequence. The SNP probe touches only at each pixel, then the tip is pulled away from the sample between measurements, thereby lowering the risk of tip and sample damage.
An SNP measurement system includes a force sensor comprising a compact (10xc3x975 mm) silicon balance beam probe assembly with an attached probe and tip with integral capacitive sensors for force and location detection such as described in U.S. Pat. No. 5,307,693 to Griffith et al. entitled xe2x80x9cForce-sensing system, including a magnetically mounted rocking element.xe2x80x9d The probe is an etched glass fiber mounted on a glass tab using an epoxy. The assembly is held in position by a base-plate magnet and pivots on a pair of precision ball bearings. The assembly is positioned by a piezoceramic scan actuator attached to its base. Position is sensed in x, y, and z axes by three sets of capacitance sensors. This configuration allows the balance beam assembly to be moved quickly and easily, either manually or by an automated probe exchanger. Electronic control keeps the beam in balance, maintaining tip orientation within 0.1xc2x0 during the measurement scan. Data from the capacitive sensors are fed back to eliminate hysteresis, nonlinearity, and drift, which are characteristic of piezoceramic scanners. The x, y, and z data are combined to define the measured surface profile.
During a measurement, with the balance beam balanced by voltages on the capacitor plates, the probe tip senses the sample surface. When the force that the sample surface exerts on the probe tip upsets the balance (typical force sensitivity  less than 50 nN), the voltage is changed on the balance capacitor to re-establish beam balance. This re-balancing voltage is directly proportional to the force sensed by the probe. Beam balance is maintained so that the geometrical relationship between the tip and the sample surface stays the same, including the tip""s angle of contact. The motion is such that the tip always contacts the sample surface at a constant angle relative to the plane of the balance beam. This contact angle is determined by the tilt of the sample and is typically 90xc2x0.
There is no practical all-purpose probe shape optimal for all types of surface features, so SNP is optimum when equipped with an onboard library of characterized tips for different applications. For example, the measurement of a deep feature diameter at both the bottom and top of a sidewall is best done with a cylindrical high-aspect-ratio tip. A straight-sided tip is most often used as it makes it easy to subtract the tip contour from the raw data. Typical dimensions for probes are 100-250 nm in diameter, with an overall length of 300-1500 nm. Probes are fabricated separately from the sensor, which allows for maximum flexibility of probe and tip material and shape. Automated probe assembly exchange is facilitated by the relatively large size of the balance beam assembly and the simple magnetic attachment to the scanning probe assembly.
As with any SPM, one difficulty is in the determination of the location of the probe tip""s apex point and proximal point with respect to the sample surface. For cantilever based AFMs, the determination is difficult because the substrate blocks the view of the exact location of the tip. In the SNP, the use of a glass fiber as the probe allows the determination of the tip location by slight levels of light refraction on the sample surface. While this helps in locating the tip, it is still difficult to determine where the tip is located and can take a long time or require the use of a complicated process due to the orientation of the glass fiber with respect to the silicon plate. Furthermore, once the probe tip location is found, it becomes difficult to then image the sample surface as the light from the long working distance microscope has to propagate through the glass tab, epoxy, and the length of the fiber.
In view of the foregoing background, it is therefore an object of the invention to improve the determination of the location of the probe tip for scanning probe microscopy.
It is another object of the present invention to provide a probe tip which allows the location of the probe tip to be more easily determined.
These and other objects, features, and advantages in accordance with the present invention are provided by a scanning probe microscope including a sensor head adjacent a stage for holding a sample, a scanning actuator for positioning the sensor head relative to the sample, and a probe carried by the sensor head. The probe preferably comprises a base connected to the sensor head, a shank extending from the base at an angle offset from perpendicular to the base, and a tip connected to a distal end of the shank for contacting the sample. The angle is preferably in a range of 5 to 20xc2x0. The tip is preferably laterally offset from the base to permit viewing of the tip location without interference from the shank and the base. Thus, the location of the probe tip may be more easily determined.
Preferably, the shank and the tip are integrally formed as a monolithic unit and comprise an optical fiber. Also, the tip preferably extends from the distal end of the shank at an angle offset from an axis of the shank, and/or extends from the distal end of the shank in a direction generally parallel to an imaginary line extending perpendicularly from the base. An adhesive may secure the shank to the base, the tip has a reduced diameter relative to the shank, and the base preferably comprises a glass plate. Furthermore, the scanning probe microscope may include an optical viewer for viewing a tip location from above.
The objects, features, and advantages in accordance with the present invention are also provided by a method of making the probe including the steps of forming a probe shank and connecting the probe shank to a base to be connected to the scanning probe microscope, and forming a tip at a distal end of the probe shank for contacting the sample. The tip extends from the probe shank at an angle offset from an axis of the probe shank. Forming the tip preferably comprises chemically etching the probe shank and/or etching the probe shank with a focused ion beam (FIB).
The objects, features, and advantages in accordance with the present invention are also provided by a method of scanning a sample with a scanning probe microscope. The method includes the steps of mounting a probe to the scanning probe microscope, positioning the probe relative to the sample while viewing a tip location from above, and measuring the sample with the probe. The probe preferably comprises a base connected to the scanning probe microscope, a shank extending from the base at an angle offset from perpendicular to the base, and a tip connected to a distal end of the shank for contacting the sample. The tip is preferably laterally offset from the base to permit viewing of a location of the tip without interference from the shank and the base.